Electrically Tunable Quantum Control for Interlayer Exciton Superlattices with Metasurface Nanoelectrodes

Atomically-thin transition metal dichalcogenide (TMDC) monolayers have attracted significant and broad interest in the last decade. They provide a clean two-dimensional system for the investigation of low-energy optical excitations because of the enhanced quantum effects due to the reduction of the available states in the system. Specifically, an interlayer exciton state can be formed when stacking two different types of TMDC monolayers into a heterobilayer with the type-II band alignment, where electrons and holes are separated in two different layers. As compared with their intralayer counterpart, interlayer excitons feature electrically tunable energy levels by a linear Stark effect and display much longer lifetimes since their recombination rate is limited by quantum tunneling.<br/><br/>By leveraging the state-of-the-art nanofabrication technology, we explore that, instead of relying on the passive built-in potential in moiré heterobilayers, we can create extrinsic but dynamically tunable electrostatic potential distributions for interlayer excitons [1] using the metasurface nanoelectrodes. Specifically, when applying an external bias between the top and bottom electrodes, the topography of the metasurface electrode can be imprinted into the potential of the interlayer exciton state via the non-uniform vertical electric-field distribution across the heterobilayer plane. This opens up opportunities to spatially control the energy of interlayer excitons down to the nanometer scale. For instance, our simulations show that an electrode with patterned nano-holes (diameter ~ 25 nm) can trap interlayer excitons in quantum wells with the radius of the center of mass wave function down to 3 nm. The depth of the quantum wells can also be scaled with the applied external vertical electric field up to ~ 50 meV.<br/><br/>To verify the above-described concept, a misaligned MoSe₂/WSe₂ heterobilayer (twist angle ~ 8°) encapsulated with thin hBN flakes (&lt; 10 nm) is transferred onto a 285 nm thick SiO₂/Si substrate, where a periodic grating-like structure (period <i>p</i> = 100 nm, trench width <i>w</i> = 50 nm, trench depth <i>d</i> = 50 nm) is patterned on the top surface. A graphite top electrode is also applied for the critical non-uniform vertical electric field generation. Cryogenic photoluminescence measurements confirm the validity of this concept by observing the trapped interlayer exciton state spectroscopically with the potential depth electrically tuned continuously from 0 to ~ 20 meV. Related power-, position-, and temperature-dependent measurements have been conducted to reveal the properties and interactions of the confined interlayer excitons.<br/><br/>Such prominent confinement leads to considerable exciton-exciton on-site interaction [2] where its Hamiltonian can be described using a Bose-Hubbard model. With that, we can explore the possible collective behavior of interlayer excitons by gradually tuning the depth of the quantum wells, which can be experimentally characterized by an electrically tunable interlayer exciton diffusivity.<br/><br/>[1] Nanoscale Trapping of Interlayer Excitons in a 2D Semiconductor Heterostructure. Nano Lett. 21, 5641−5647 (2021).<br/>[2] Single-exciton trapping in an electrostatically defined two-dimensional semiconductor quantum dot. Phys. Rev. B 106, L201401 (2022).